Frequency-modulated continuous-wave lidar using I/Q modulator for simplified heterodyne detection

Abstract

A frequency-modulated continuous-wave (FMCW) lidar is demonstrated with heterodyne detection. The lidar transmitter utilizes an electro-optic I∕Q modulator for the first time to generate carrier-suppressed and frequency-shifted FM modulation. This eliminates the need for an acousto-optic frequency shifter commonly used in heterodyne lidar transmitters. It also allows the use of a much wider modulation bandwidth to improve the range resolution. The capability of complex optical field modulation of the I∕Q modulator provides an additional degree of freedom compared with an intensity modulator, which will benefit future lidar applications. © 2012 Optical Society of America OCIS codes: 060.2300, 280.3640, 120.4640.Lidar systems have been widely used for measuring range, velocity, vibration, and air turbulence [1][2][3]. Coherent detection has become a major detection mechanism in lidar systems because of the much improved receiver sensitivity compared with direct detection. While homodyne detection is susceptible to the phase noise of the optical carrier and the associated signal fading, heterodyne detection is a more robust detection scheme for many practical lidar systems [4,5]. In a heterodyne receiver, mixing between the optical signal and the local oscillator (LO) shifts the optical signal to an intermediate frequencyIn practical implementation of a lidar system, the signal and the LO are usually split from the same laser. While the signal is modulated by an electro-optic modulator, the LO has to go through an acousto-optic modulator (AOM) to create the required frequency shift f IF . In addition to the increased complexity, the frequency shift of an AOM is usually not more than 1 GHz. Although the bandwidth of a practical electro-optic modulator can be higher than 40 GHz, to avoid spectral aliasing, the usable modulation bandwidth of a heterodyne detection lidar is limited by the IF frequency, which is determined by the AOM. In recent years, the rapid advance in coherent fiber-optic communication has created high-speed electro-optic in-phase/ quadrature (I∕Q) modulators, which are capable of modulating the amplitude and the phase of the optical carrier independently. Despite the popular application of I∕Q modulators in optical communication systems, their application in lidar has not been reported. In this paper, we demonstrate a frequency-modulated continuous-wave (FMCW) lidar system using heterodyne detection. An I∕Q modulator is used to simultaneously generate the linear FM chirp required for pulse compression and the IF frequency shift required for heterodyne detection. Therefore, the optical frequency of the LO does not have to be shifted, and the AOM is not required. Based on a commercial transmitter originally designed for a 10 Gb∕s optical communication system, the lidar system allows unprecedented chirping bandwidth to ensure the fine range resolution, while the optical system configuration is simplified.For an FMCW lidar system [6], a linear frequency chirp is applied across each optical pulse. The r...

Nanophotonic coherent imager

“…Although 3D imaging systems have been studied, they are usually realized either as single pixel detectors [1][2][3][4] or as bench-top systems with simple detector arrays [5][6][7]. Silicon nanophotonic processes use similar steps as their electronic counterparts, leading to higher yields, and compatibility with electronic integrated circuits.…”

Complex-optical-field lidar system for range and vector velocity measurement

“…This limits the range resolution. This limitation can be overcome by using an electro-optic I/Q modulator which is able to provide carrier-suppressed single-sideband modulation to generate a much higher f IF for coherent heterodyne lidar system and achieve unprecedented range resolution [10].…”

“…In fact, an I/Q modulator allows independent RF signals to be loaded onto the upper and the lower optical sidebands [11]. Generating high frequency f IF in [10] only addresses one of the two optical sidebands. The other sideband is not used so that only range information was measured in a fiber delay line in reference [10].…”

“…Generating high frequency f IF in [10] only addresses one of the two optical sidebands. The other sideband is not used so that only range information was measured in a fiber delay line in reference [10].…”

“…After photodetection, electrical signals are sampled by a pair of high speed analog-to-digital converters (ADC) before they are analyzed through digital signal processing. To create a wideband chirp waveform which linearly sweeps from f 1 to f 2 on the upper optical sideband, and a constant frequency modulation at f CW on the lower optical sideband, the two voltage waveforms used to drive I and Q arms of the I/Q modulator are [10],…”